What genetic process is occurring in a puff of a polytene chromosome? How do we know this experimentally?

Verified step by step guidance

1. Understand that a puff in a polytene chromosome represents a region where the chromatin has decondensed, indicating active gene expression.
2. Recognize that the genetic process occurring in a puff is transcription, where RNA polymerase synthesizes RNA from the DNA template.
3. Recall that polytene chromosomes are large, banded chromosomes found in certain tissues (like salivary glands of Drosophila), allowing visualization of chromosomal activity under a microscope.
4. Know that experimental evidence for transcription in puffs comes from autoradiography experiments where radioactive RNA precursors (like 3H-uridine) are incorporated into newly synthesized RNA localized at the puff regions.
5. Conclude that the presence of newly synthesized RNA at the puff sites confirms that transcription is actively occurring in these regions of the polytene chromosome.

Key Concepts

Polytene Chromosomes

Polytene chromosomes are oversized chromosomes found in certain tissues like the salivary glands of Drosophila. They result from repeated rounds of DNA replication without cell division, producing many aligned chromatids. Their distinctive banding patterns and puffs allow visualization of gene activity at the chromosomal level.

Chromosomal Puffs and Gene Transcription

Chromosomal puffs are regions where the chromatin decondenses, indicating active gene transcription. These puffs represent sites where RNA polymerase and transcription factors are engaged in synthesizing RNA, making them visible markers of gene expression in polytene chromosomes.

Experimental Evidence for Transcription in Puffs

Experimental methods such as autoradiography with radioactive RNA precursors and in situ hybridization have shown that puffs correspond to sites of active RNA synthesis. Incorporation of labeled nucleotides into RNA at puff sites confirms that transcription is occurring there.